1. Field of the Invention
The present invention relates to an ink composition. More particularly, the invention relates to an ink composition that resists bleeding between a printed image and original color, exhibits improved water fastness of the printed image, and exhibits improved dry and wet rub fastness.
2. Description of the Related Art
In general, coloring agents embody their inherent colors by selectively absorbing or reflecting visible light. The coloring agents are classified as dyestuffs and pigments.
Dyestuffs are used in many materials to be dyed, such as fibers, leathers, furs, and papers thereby providing considerable fastness to daylight washing, friction, etc. Pigments are coloring matters in the form of particulates. The particulates are not absorbed into the material to be dyed but are adhered to the surface of the material to be dyed, by physical means (e.g., adhesion, etc.) thereby providing the inherent color.
Dyestuffs are dissolved in solvents such as water, alcohol or other organic solvents and applied directly or indirectly to the substrate being dyed. Pigments are generally insoluble to most solvents, and thus it is most important to disperse a pigment particulate homogeneously in a solution, suspension or dispersion. The pigments are typically maintained in a stable dispersed state to inhibit re-aggregation of the pigments.
Water-soluble dyestuff-type inks are very superior in long-term storage stability, maintaining homogeneity of the ink, and retaining clear color and brightness. However, water-soluble dyestuff type inks exhibit poor water fastness, and light resistance.
Pigment-type inks have high optical density (OD), superior water fastness and light resistance, and exhibit little bleeding between colors. However, the color clearness is poor and its long-term storage stability is low compared to the dyestuff-type inks. Also, the images printed by the pigment-type inks have poor dry and wet rub fastnesses.
When printing in colors (multicolor printing) with dyestuffs or pigments, bleeding at interfaces of each color occurs, and thus clearness of images is reduced. The rub fastness and color fastness can be improved by decreasing mobility of the coloring agent on receptors such as a paper. Examples of ink compositions for ink-jet printers comprising a pigment, a water-soluble resin and a specific alcohol are disclosed in U.S. Pat. Nos. 5,172,133 and 5,529,616.
However, the ink-jet ink used in jet printing is applied through nozzles, so that it is necessary to maintain a low viscosity. Thus, the amount of the polymer binder used in the ink is limited. A minimum amount of the binder is required to sufficiently bind pigment particles to the substrate. When the amount of the binder is too low, the ink exhibits unsatisfactory rub fastness and color fastness.
Another approach to improve rub fastness and color fastness by decreasing mobility of a coloring agent on receptors, by chelating a metal ion as disclosed in U.S. Pat. No. 4,694,302. Rub and color fastness are also purported to be improved by the reaction of a cation and an anion as disclosed in U.S. Pat. No. 5,623,294, and by a reaction of polymer as disclosed in U.S. Pat. No. 5,629,359. However, the method of chelating a metal ion has difficulty in obtaining long-term storage stability due to a reaction of each ink constituent and a metal ion contained in an ink. The method of reacting a cation and an anion also has poor performance due to a reaction between the substrate and the ions as well as a reaction of ink constituents. Also, the method of reacting a polymer has an environmental burden due to an unreacted monomer, the need for a curing apparatus and curing time, and the difficulty in obtaining long-term storage stability.
Thus, an ink composition is required to improve the color fastness such as bleeding resistance, rub fastness, water fastness, etc. and to provide improved quality of a printed image as well as long-term storage stability.